It looks like a concretion of red crag shelly sandstone that has incorporated a gastropod during its formation. Concretions are formed where an area of sediment is further cemented together by an accumulation of additional minerals, making it harder than the surrounding rock so that it erodes out of it intact.

The shell has been exposed and broken during weathering, making it difficult to identify from guides which usually show the exterior of the shell only. An expert might be able to have a go at identifying it from the general proportions and the shape of the columella (the column around which the gastropod shell coils).

I think your shell could be encrusted by a hydoid called Hydractinia echinata. This hydroid has a calcarous skeleton and often encrusts shells especially gastropods. If the shell is occupied by a hermit crab this is also and advantage as it gets a free ride. You can just make out pores in the detail of your photograph. Both the shell and the hydoid are probably Pleistocene fossils from the Red Crag so about 2.6 million years old.